1. Field of the Invention
The present invention relates to semiconductor fabrication, and more particularly to systems and methods for monitoring and controlling the operation of extreme ultraviolet (EUV) light sources used in semiconductor fabrication.
2. Description of Related Art
Semiconductor fabrication typically involves dozens, or even hundreds of individual operations. In general terms, these operations can be classified as: layering, patterning, doping, and heat treatments. Among these, patterning (also referred to as “lithography”) is considered to be the most critical operation because it sets the physical dimensions of the resulting devices on the semiconductor wafer.
Patterning comprises a series of steps by which selected portions of material deposited on the wafer surface are removed, thus leaving a “pattern” thereon. The sequence of steps may be described as follows. First, a pattern is formed on a photomask or reticle—e.g., a glass or quartz plate having a thin layer of chrome thereon. Then, the photomask is imaged or printed onto a layer of photoresist deposited on the wafer. Etching agents remove portions of the wafer not covered by the photoresist, and the photoresist itself is removed in subsequent steps. The transfer of the pattern from the photomask onto the photoresist is performed using some form of light source or electromagnetic radiation.
The wavelength of the light source used during the lithography process is directly proportional to the size of the features that can ultimately be fabricated on the semiconductor wafer. Hence, the continuous desire to create increasingly small semiconductor devices has created a need for light sources capable of emitting very short wavelength radiation. One such light source is the Extreme Ultraviolet (EUV) plasma source. EUV radiation may be generated in the 13.5 nm wavelength range by a plasma-based process whereby a fuel material such as xenon, lithium, indium, tin, etc. is heated to high temperatures. This intense heat may be achieved through the use of a focused laser beam, plasma pinch electrodes applying high-energy, short-duration pulses, or the like.
Light produced by EUV sources provides a printing resolution in the order of nanometers. Extremely small features down to 25 nm have been satisfactorily imaged. It is, however, very difficult to monitor the performance of these sources. U.S. Patent Application Publication No. 2003/0058429 discloses an apparatus that attempts to measure the energy of an EUV photon system. The apparatus is designed to detect the in-band power of a pulsed or continuous source—i. e., at a wavelength between 11 and 15 nm—and filtering out the out-of-band radiation by several orders of magnitude.
The inventors hereof have recognized several significant drawbacks with existing EUV monitoring systems such as the one referred to above. First, multi-layer mirrors used in these systems quickly lose their reflectivity over time and frequently need to undergo expensive and time consuming calibration procedures, thus decreasing the overall productivity. Moreover, existing EUV systems must perform measurements in a high vacuum, which further adds to the cost and complexity of the system.
The shortcomings mentioned above are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques for monitoring energy from EUV sources. The problems listed above are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory, and that a significant need exists for the systems and methods described and claimed herein.